Table i.xelecteolyte c composition

ABSTRACT

1. A RECHARGEABLE THERMAL ELECTROCHEMICAL CELL HAVING AN ANODE, A CATHODE, AND AN ELECTROLYTE IN CONTACT WITH SAID ANODE AND CATHODE, WHEREIN, IN THE CHARGED CONDITION: (A) SAID ELECTROLYTE COMPRISES ALKALI METAL FLUOMETALATE, WHEREIN SAID FLUOMETALATE (METALATE) IS FLUOBERYLLATE (BERYLLATE) OR FLUOZIRONATE (zIRCONATE); (B) SAID ANODE CONSISTS ESSENTIALLY OF A METAL EQUAL IN ACTIVITY TO OR LESS ACTIVE IN THE ELECTROMATIVE SERIES THAN THE METAL OR SAID FLUOMETALATE (METALATE); AND (C) SAID CATHODE COMPRISES NICKEL FLUORIDE, SILVER FLUORIDE, OR COPPER FLUORIDE.

Nov. 26, 191 5 sEN o F RI. 2, 250

RICHLRGEABLB BLECTRO-CHEIICAL CELL HAVING COUPLE-X IITAL FLUORIDE ELECTROLYTE Original Filed March 12, 1971 7 Sheets-Sheet DOTTED LINES REPRESENT INCOMPLETELY DEFINED PHASE BOUNDARIES AND ALKEMADE LINES TEMPERATURE (C COMPOSITION (mOIB NuF- BQF 595 i 570 LiF 800 750 700 649 700 750 800 850 900 950 NcF 980 THE SYSTEM LiF-NGF- BeF INVENTOR Seymour SPm/Pmff BY I I a ULALL ZILJ Q INK; ATTORNEY Nov. 26, 1974 s. SENDEROFF Re. 28, Z50

,. RECHARGEABLE ELECTED-CHEMICAL CELL HAVING COMPLEX IBTAL FLUORIDE ELECTROLY'I'E Original Filed March 12, 1971 7 Sheets-Sheet 2 TEMPERATURE IN C COMPOSITION IN mole "/0 THE SYSTEM NoF-KF-Be-F \NVENTOR Seymour Senderaff BY I ,4

ATTOl QNEY NOV; 26, 1974 s, SENDERQFF Re. 28, 250

REGHARGEABLE ELECTED-CHEMICAL CELL HAVING COMPLEX METAL FLUORIDE ELECTROLYTE original Filed arch 1971 7 Sheets-Sheet g o cn \i 0 P3 2 2 l azax-a m b m O N Z a 3 E a 8 5 5 LL m k 0 (:6 X b 8 m (a 3 u 3 8 dNZ-B E o M H n a e-5x2 Q o o Q o o O 0 O 8 g E 8 a 2 m N (09) EIHfilVHEdWELL Q5 mvzmon Seymour Sendaroff n Lk Y1'l" i I WW f1) ATTORNEY Nov. 26, 1974 s. SENDEROFF Re. 28, 250

nzcmncmmm ELECTED-CHEMICAL CELL ruwwe courLEx IlTAL rwonzm: ELEc'rnom-m Origin Filed Ilarch 12, 1971 7 Sheets-Sheet 4 r t l i H INVENTOR Seymour Senderoff Nov. 26, 1974 s. SENDEROFF Z50 RECHARGEAHLE ILECTRO-CHEIICAL CELL HAVING COMPLEX IBTAL FLUORIDE ELECTRDLYTE Original Filed March 12, 1971 7 Sheets-Sheet 5 I: 6 2.0 v 30 Ohm loud Lu 2 *3 O TIME, MINUTES DISCHARGE CURVE OF Be,NiF CELL IN SOLUTION II AT mA/cm (UPPERIond .3OOm1 1/cm (LOWER) TEMP, 400C 0.2 sec.

CHRONOPOTENTIOGRAM OF CHARGE AND DISCHARGE OF NICKEL IN SOLUTION I at 2OOmA/cm The 0.2 poIorizuIion shown Is accounted for by IR drop in the eIecIroIyIe. Temp,400C

Seymour Senderoff ATTOR N EY Nov. 26, 1974 s. SENDEROFF RECHAHGEABLB BLECTRO'CHEIICAL CELL HAVING COMPLEX I METAL FLUORIDE ELECTROLYTE Original Filed larch 12, .1971

7 Sheets-Sheet 6 0.2 sec. CHRONOPOTENTIOGRAM OF CHARGE AND DISCHARGE OF Cu IN SOLUTION I AT 50 mA/cm Temp. 370C IIII IIII

III TIITIIIT 1111 IIII lI/J H In] lllI JIII IIII' II [III II IIII IH I I! II 0.2 sec. CHRONOPOTENTIOGRAM OF CHARGE AND DISCHARGE OF Cu IN SOLUTION I AT 200 mA/cm Temp. 370C INVENTOR Seymour Senderoff X HI ATTORNEY Nov, 26, 1914 s. SENDEROFF Re. 28, Z50

RICHARGEABLE ELECTED-CHEMICAL CELL HAVING COIPLEX .ITAL FLUQRIDE ELICTROLYTE 1971 7 Sheets-Sheet 7' Original Filed larch 1.2

Illl {TIT 0.2 sec.

ll IIll III] III III] [III CHRONOPOTENTIOGRAM OF SILVER CHARGED AT ISOmA/ cm AND DISCHARGED AT SOmA/cm IN SOLUTION I Temp,

F/GB.

0.2 sec.

lll IIIIIIIIIIII [III lllIlIll lIlll Ill' IIII IIII IIII II[I IIII IIII IIII IIII IIIT CHRONOPOTENTIOGRAM OF CHARGE AND DISCHARGE OF Ag IN SOLUTION II AT IOOmA/cm SHOWING SINGLE STEP PROCESS Temperature 400 C INVENTOR Seymour Senderoff BY ATTORNEY United States Patent 28,250 RECHARGEABLE ELECTRO-CHEMICAL CELL HAVING COMPLEX METAL FLUORIDE ELECTROLYTE Seymour Senderofi, Fairview Park, Ohio, assignor to Union Carbide Corp., New York, N.Y.

Original No. 3,725,128, dated Apr. 3, 1973, Ser. No. 123,570, Mar. 12, 1971. Application for reissue May 31, 1973, Ser. No. 365,685

Int. Cl. H01m 36/00 US. Cl. 136-6 LF 21 Claims Matter enclosed in heavy brackets IQ] appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE Rechargeable electrochemical cells are disclosed having complex metal fluoride electrolytes comprising an alkali metal fluoberyllate or fluozirconate. In the charged condition, the anode is a metal not greater in activity in the electromotive series than beryllium or zirconium, respectively, and the cathode is the fluoride of a metal less active in the electromotive series than the metal of the anode. A preferred cell has, in the charged condition, a beryllium anode, a nickel fluoride or silver fluoride cathode, and an alkali metal fluoberyllate electrolyte.

The invention relates to rechargeable electrochemical cells having alkali metal fluoberyllate or fluozirconate electrolytes. In one aspect, the invention relates to rechargeable thermal batteries having an alkali metal fluo beryllate or fluozirconate electrolytes.

In view of the high energy density and high rate capabilities of thermal batteries, there has been considerable interest in such batteries as candidates for long lived secondary batteries for use in electric cars. To date, however, most thermal batteries in use today are reserve primary batteries that are activated by heat and discharged at a high rate in a few seconds or minutes. There has, however, been considerable research and development work done on four types of secondary thermal batteries. These types are the lithium/chlorine cell, the sodium/ sulfur cell, the sodium/air cell, and the lithium/chalcogen (tellurium, selenium, or sulfur) cells.

Each of these types of cells has certain important disadvantages. For instance, the lithium/chlorine cell uses molten lithium, which is highly corrosive at operating temperature (about 650 C.), and pressurized chlorine gas. Therefore, expensive corrosion resistant matcrials must be employed, and means must be found to substantially eliminate the possibility of chlorine leakage. The electrolyte employed in the sodium/sulfur cell is beta aluminum oxide, a ceramic that is sensitive to mechanical and thermal shock. Also, the relatively high resistivity of beta aluminum oxide limits this cell to modest current densities. The current densities available from the sodium/ air cell are relatively low, owing to sodium diffusion limitations and loss of voltage at the interface between the fused salt electrolyte and the sodium electrode. One severe design limitation common to all of these cells is that they must provide for the utilization of liquid metal anodes and either liquid or gaseous cathodes. A review of thermal cells, and utilities therefor, is found in an article by Cairns et al., High-Temperature Batteries, Science 164, Number 3886, 1347 (June 20, 1969).

Hunt et al., in US. Pat. No. 3,498,843 described a thermal cell having an electrochemically active metal anode (e.g., magnesium or sodium), a transition metal fluoride cathode, and an electrolyte consisting of alkali metal fluoride, alkaline earth metal fluoride, or mixtures See thereof. There is no mention in the patent of rechargeability of the cells described. Indeed, since most of the cathode materials disclosed are known to be thermally unstable (e.g., K CuF K MnF K NiF and the like), the Hunt et al. cells apparently cannot be employed as secondary batteries.

The present invention is based upon the discovery that certain thermal cells utilizing fluoberyllate or fluozirconate electrolytes and which have solid electrodes at use temperatures, are rechargeable. These cells therefore possess valuable utility as secondary batteries. Accordingly, it is an object of the invention to provide rechargeable thermal cells.

Another object of the invention is to provide an electrochemical cell having a fluoberyllate or a fluozirconate electrolyte.

Still another object of the invention is to provide rechargeable thermal cells having electrodes which are solids at the temperature of operation.

A further object of the invention is to provide an electrochemical cell having a beryllium anode, an alkali metal fluoberyllate electrolyte, and a metal fluoride cathode.

A still further object of the invention is to provide an electrochemical cell having a zirconium anode, an alkali metal fluozirconate electrolyte, and a metal fluoride cathode.

An additional object of the invention is to provide a secondary thermal electrochemical cell having relatively stable components.

These and other objects and advantages of the invention will be apparent from the following description, taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a phase diagram of the system lithium fluoridesodium fluoride-beryllium fluoride;

FIG. 2 is a phase diagram of the system sodium fluoride-potassium fluoride-beryllium fluoride;

FIG. 3 is a phase diagram of the system potassium fluoride-beryllium fluoride;

FIG. 4 is a schematic elevation, partly in section, of a test cell to which the principles of the invention can be applied;

FIGS. 58 and 10 are chronopotentiograms of various cell systems embodying the principles of the invention; and

FIG. 9 is a graph of cell voltage vs. time of a cell embodying the principles of the invention, at two different load conditions.

Broadly, the rechargeable electrochemical cell of the invention has an anode, a cathode, and an electrolyte in contact with the anode and cathode, wherein, in the charged condition, the electrolyte comprises alkali metal fluometalate wherein the metalate is beryllate or zirconate, the anode comprises a metal equal in activity to or less active in the electromotive series than the metal of said metalate, and the cathode comprises the fluoride of a metal less active in the electromotive series than the metal of the anode.

An important feature of the present invention resides in the provision of an electrolyte comprising an alkali metal fluoberyllate or fluozirconate. Referring first to the fluoberyllate system, an alkali metal fluoberyllate can be considered to be a mixture of alkali metal fluoride and beryllium fluoride (BeF Thus, this electrolyte will contain BcF plus one or more of, for instance, lithium fluoride, sodium fluoride and potassium fluoride. Cesium and rubidium fluorides could also be piesent, although systems containing substantial amounts of these materials often tend to have higher melting points; which would detract from the commercial attractiveness of such systems in many cases.

Preferred electrolytes include those that have relatively low melting points, and especially those whose melting TABLE I.-ELECTROLYTE COMPOSITION (MOL. PERCENT) Solution I II III Composition:

BeFa 37.5 47. 5S LiF 31. 5 NaF. 31.0 26. 3 KF 26.3 42 M1". C 315 350 360 Resistivity (90th.) l 3 5 1 Approximate values (125% at 450 (2.).

FIGS. 1, 2, and 3 display, respectively, phase diagrams of the mixtures lithium fluoride-sodium fluoride-beryllium fluoride, sodium fluoride-potassium fluoride-beryllium fluoride, and potassium fluoride-beryllium fluoride. These phase diagrams can be employed in a manner known in the art to aid in selecting an appropriate electrolyte composition having the desired melting point and beryllium fluoride concentration.

The factors to consider in selecting a particular electrolyte for use in the invention include one or more of the following:

As a general rule, the lower the melting point of the electrolyte the better. However, when use as a heat activated reserve cell is intended, the melting point should be higher than any ambient temperature that can reasonably be expected to be encountered. Electrolyte melting points of between about 350 C. and about 600 C. are most frequently encountered. Electrolyte melting points below about 400 C. are preferred in most cases. However, since cells embodying the principles of the invention have been operated at quite high temperatures, for instance, up to about 800 C., electrolyte melting points in excess of about 600 C. are permissible, in some cases.

The exact concentration of beryllium fluoride desired in the electrolyte will vary from one system to another, depending upon the nature of the materials in the electrolyte. An important factor to consider in selecting the proportion of beryllium fluoride is the maintenance of the melting point of the electrolyte below a preselected point throughout the entire range of electrolyte compositions prevailing throughout the charge-discharge cycle of the cell. Thus, referring to FIG. 3, which shows the phase diagram for the system KF-BeF when employing a mixture of potassium fluoride and beryllium fluoride as the electrolyte, along with a beryllium anode, the concentration of BeF in the electrolyte when the cell is in the charged condition is preferably from about 52 to about 60 mole percent, in order to keep the melting point of this electrolyte below about 400 C. during the chargedischarge cycle of the cell. In calculating the amount of BeF that will be added to the electrolyte during cell discharge, normally not more than about 75 percent of the anode will dissolve in the electrolyte during discharge. The preferred amount of beryllium fluoride to use in other electrolyte systems can be readily determined in an analogous manner by resort to the appropriate phase diagram, such as those shown in FIGS. 1 and 2. (Phase diagrams of mixtures of various metal fluorides, including such mixtures containing Belor ZrF are found in Phase Diagrams of Nuclear Reactor Materials, R. E. Thoma, ed., ORNL-2548).

The function of the BeF in the electrolyte is to supply beryllium for the anode in order to impart rechargeability to the cell. The alkali metal fluoride functions to lower the melting point and increase the conductivity of the electrolyte. In theory, the fluoride of any metal higher than beryllium in the electromotive series can be present in the electrolyte in place of some or all of the alkali metal fluoride. However, such metals include thorium (which is expensive and radioactive), rare earths (which are expensive), and alkaline earth metals (whose fluorides have very high melting points). The indicated disadvantages of these materials would appear to suggest that their use as major components of the electrolyte would be undesired for most commercial applications.

A table displaying the standard elcctromotive forces for solid and molten metallic fluorides, which is the EMF series that is applicable to this invention, is found in Hamer et al., EMF Forces For Cells Containing Halides," J. Electrochem. Soc. 112, 750, 751 (1965).

The anode material is a metal that is equal in activity or less active in the electromotive series than beryllium. Preferably, the anode material is beryllium. However, other metals such as titanium or aluminum can be employed, although the voltages of cells using these metals would be relatively low.

The cathode material is a metal fluoride, wherein the metal is lower in the electromotive series than the metal employed in the anode. In addition to being lower in the electromotive series than the anode metal, it is preferred that the metal of the cathode not be beryllided by the electrolyte, which would drastically reduce the cell voltage. Nickel, copper and silver have been found to be satisfactory metals to be employed in the cathode. Nickel and silver are preferred, since nickel gives a one step discharge with Solutions 1, II, and lll as electrolytes (cf.see Table I, above), and silver gives a one step discharge with Solutions II and III, whereas copper gives a two step discharge with all three of these solutions.

In order to determine the particular cathode material to employ with any particular cell, a number of factors are considered. First, the cathode material is preferably only slightly soluble in the electrolyte at the temperature of operation. Thus, at use temperatures, solubilities of the cathode material in the electrolyte [eleytrolyte] of from about l0 to about 10' weight percent (based on weight of electrolyte) are preferred in most cases. Solubilities less than about 10' are not often encountered in inorganic systems of this type. If the solubility of the cathode material is substantially higher than l0- the cathode material would react with the anode unless an efficient separator were provided.

The reaction occurring at the cathode should be reversible. Thus, the reactions (1) metal fluoride to metal plus fluoride ion, on discharge, and (2) fluoride ion plus metal to produce metal fluoride, on charge, should proceed readily. Also, the conductivity of the metal fluoride and elemental metal derived therefrom should be adequate. Copper fluoride, nickel fluoride, and silver fluoride have been found to satisfy these requirements of conductivity and reversibility of reaction.

Referring now to the fluozirconate system, this system can typically be considered to be a mixture of zirconium fluoride (ZrF with alkali metal fluorides. A preferred composition in the charged state contains 10 mol percent sodium fluoride, 48 mol percent potassium fluoride, and 42 mol percent zirconium fluoride. This mixture has a melting point of 385 C. Pages 52-67 of R. E. Thoma, ORNL2548 (op. cit., supra) display phase diagrams of various alkali metal fluoride-zirconium fluoride systems which can be used as electrolytes in the invention. As with the fluoberyllate systems, electrolyte melting points below about 600 C., and preferably below about 400 C., are most frequently employed.

The factors to be considered in determining the concentration of zirconium fluoride, and the exact nature of the components in the electrolyte, are analogous to those discussed above with respect to the fluoberyllate system.

In the fluorzirconate system, the anode is preferably zirconium, although other materials, such as titanium, which have activities slightly less than zirconium in the electromotive series can be employed.

The cathode is a metal fluoride, the metal being lower in the elcctromotive series than the metal of the anode. Among the metal fluorides that have been successfully employed as the cathode in the fluozirconate system, there can be mentioned copper fluoride, silver fluoride, nickel fluoride, and the like. Nickel fluoride is preferred. The factors to be considered in selecting the specific cathode material are analogous to those described above with respect to the fluoberyllate system.

The electrochemical cells of this invention can be constructed by procedures that are known in the art. As a general rule, it is desirable to elcctroform the anode on a support such as expanded metal, a metal screen, or a metal spiral. Inert metals such as nickel are employed as the anode support. It is preferred to electroform the anode because the resultant anode has a higher surface area, is more active, and is also in the unavoidable form that will be obtained after the first discharge-charge cycle.

The cathode can be made by conventional procedures, such as by the known preparation of pasted electrodes. A general discussion of pasted electrodes is found in G. W. Vinal, Storage Batteries, pages 27-46, Wiley and Son (1955). A specific method of preparing pasted electrodes that has been found desirable for use in the invention is given in a paper Non-Aqueous Lithium-Nickel Halide Batteries, presented by A. E. Lyall and H. N. Seiger at the Intersocicty Energy Conversion Conference, Los Angeles, Calif. (September 1966).

The cells of the invention are preferably hermetically sealed for commercial applications. In the laboratory, it

is usually more convenient to blanket the cell under an inert atmosphere such as argon or helium. The operating temperature of the cell is preferably sufliciently above the melting point of the electrolyte so that the viscosity of the electrolyte is not so high that it interferes with efficient operation of the cell. The examples below illustrate such conditions.

The thermal cells of the invention have wide utility. For instance, they can be used as heat activated reserve [reverse] cells for ordnance (e.g., to operate fuses, and the like). They can be used to power electric vehicles such as carts, cars, and the like.

The following examples illustrate certain aspects of the invention:

EXAMPLE 1 FIG. 4 shows a schematic elevation, partly in section, of a test electrochemical cell that was constructed to demonstrate the invention. The various parts of this cell were as follows:

Legend for FIG. 4

The following materials were employed in the experiments with this cell:

Materials: BeF technical grade, was obtained from Brush Beryllium Company. It was stated to be 98.5% BeF min. with Net-0.4%, Al-0.02%, and

max. Be metal (strips), high purity, was obtained from the same company. Alkali fluorides and nickel iluoride. C. P. chemicals, were obtained from Baker or Allied Chemical Company. CuF, CuF Agl and AgF were prepared from reagent grade chemicals by standard methods. (Handbook of Preparative Inorganic Chemistry, G. Brauer, ed., vol. I, 2nd ed., translated by Scripta Technicia, Academic Press, New York, NY. (1963).)

The experimental procedure employed was as follows:

The powdered ingredients were weighed and mixed in a dry box in an argon atmosphere at a pressure of 0.2-0.5 inch of water below atmospheric. They were then charged into the crucible and envelope shown in FIG. 4, the electrodes and auxiliary equipment inserted in the plastic cover to a level which would keep them in the cold zone above the crucible and the envelope sealed with the transparent plastic cover. The assembly was then placed in a furnace in a fume hood with the temperature controlled and measured with a Chromel-Alumel thermocoupled and pyrometer controller. A mirror above the cover permitted observation of the interior of the cell. After the electrolyte was brought to the desired temperature, it was electrolyzed between beryllium electrodes to purify it partially, the originally cloudy melt becoming clear and transparent in the process. The electrolysis electrodes were than withdrawn and the test electrodes and reference electrode inserted as shown. These adjustments were made by moving the appropriate rods in the Swagelok seals without opening the cell to the atmosphere. Argon was continually passed through the system. Measurements of open-circuit potential and discharge curves were made with a Model MR Sargent recorder. Chronopotentiograms were run with l cm. metal disks and potentials measured against a Ni/NiF reference electrode in a Pyrex tube immersed in the electrolyte. (Chronopotentiograms are useful to determine the pure electrochemical characteristics of a cell, without the variations in performance and complications caused by factors such as uncontrolled mass transfer and convection. The chronopotentiogram can be considered to be a transient charge-discharge test at constant current, which measures the number of steps in the discharge, the voltage of discharge, and the extent of polarization. Chronopotentiograms are discussed in chapter 8 of New Instrumental Methods in Electrochemistry, by Paul Delahy, Interscienoe Publishers, 1954.)

The Pyrex glass was a sufliciently good conductor so that no hole or wick was required in the reference half cell. A constant current power supply and Tektronix No. 535A oscilloscope with a type D plug-in unit were used and the chronopotentiograms were photographed with a Polaroid camera attached to the oscilloscope. Solubilities were determined by equilibrating the electrolyte with the desired reagent at a temperature about 50 C. above the desired temperature, dropping the temperature to the desired level and re-equilibrating, and finally sucking the saturated solution into a Pyrex tube through a porous filter disk. The sample was allowed to freeze and was then analyzed by standard methods.

The pasted electrodes were fabricated by applying a paste of varying ratios of metal fluoride powders and a 9:1 mixture, by weight, of type 38 graphite and carbon black to expanded metal or coiled wire electrode supports. In most cases, a 1:1 mixture, by weight, of metal fluoride: carbon powder was used. The paste was prepared by wetting the fluoride-carbon mixture with methanol and mixing to a creamy consistency with a gelatin-water solution g./l.). The paste slurry was then spread on the metal substrate, dried to touch, pressed at 4000 open circuit results in reduction of open circuit potential.

pounds/inch and dried overnight in air at 100 C. Recharging restores the original potential.

RESULTS Cathodes: Most metals such as W, Mo, Fe, Zn, and Pt are rapidly beryllided by fluoberyllate electrolytes and The compositions of electrolyte shown in Table I have are difficult to charge to form cathodes. Nickel, copper thus far been studied. 5 and silver could be charged and discharged chrono- TABLE I ELECTROLYTE COMPOSITION potentiometrically With the results as shown in Table II (MOL- PERCENT) and FIGS. 5 through 8. It will be noted that both copper solution and silver gave two-step reductions with Solution I as the 1U electrolyte. In the case of copper, most of the discharge was at the lower of the two voltages. In the case of silver, the amount at each voltage varied with the charging current density. When charged at 25 ma./cm. and discharged at 50 ma./cm. almost all the discharge was at 5 360 1.8 v. However, when charged at 250 ma./cm. and dis- 3 charged at 50 ma./cm. 80 percent of the discharge was I Approximate values ($2595 at; 450 0.). at V- 'TABLE II.CATIIODE CHARACTERISTICS IN THE TERNARY BERYLLA'IE SOLUTION [Electrolyte I] Coulomhs discharge] Charging Discharge Polar- Coulemhs voltage (v.) voltage (v.) ization charge .75 s .s 501ststep... 2. 5 Ag "{2d step 2. 92

'Vs.' Be anode. Nl/NiFiIWas actually used as reference.

These were chosen not only for their conveniently A CuF pasted electrode discharged for 3 minutes at low melting points but for the additional advantage that 2.55 v. but fell to 1.85 v. where it held for 8 minutes for their melting points do not increase rapidly with added BeF The phase diagrams for the two ternary and one a fairly fiat discharge. CuF electrodes discharged between 2.0 and 1.8 volts for five minutes and then proceeded for binary system for which these compositions are eutectics 45 minutes with a reasonably fiat discharge at the latter are shown in FIGS. 1, 2, and 3 These solutions involtage. These results confirm the chronopotentiometric crease in BeF a Lewis acid, from I to ill, thus tending results indicating the low voltage step, probably to make III the most acid of the solutions. However, 40 Cu+ Cu, as the dominant one in this system. they also increase in K (a Lewis base) relative to the NiF pasted electrodes gave a flat discharge at 2.0 v. other alkali ions from I III. The solvent cation effect Coulombic ratios on charge and discharge approached would thus tend to make III the most basic of the soluunity but very little of the total material was converted tions. The acid-base characteristics can be expected to in each cycle. affect solubility of cathode materials, complex forma- Table III shows the solubility of the most promising lion, cell potentials and other properties. As seen below cathode materials in the three electrolytes of Table I. As (Table III), of these two opposing effects, that of the noted above, both BeF and K+ increase from I to III. solvent cation is the more important within the indicated Since an increase in Bel content would be expected to range of BeF variation. reduce the solubility of metal fluorides and an increase in Anode: A solid beryllium anode in Solution I against K+ content relative to the other alkali ions would be ex- 1 Ni/NiF reference electrode had a potential of 1.9 v. pected to increase the metal fluoride solubility, it is clear at 350 C. and 1.95 v. at 400 C. Polarization of the that the solubilizing effect of K+ is governing over the exsolid anode was comparatively high, i.e., 0.5 v. at 200 pected opposite eifect of BeF It also may be inferred ma./cm. and 0.1 v. at 50 ma./cm. but increasing the that while Solution I is usable with any of the three surface area by electrodepositing the beryllium from the cathode materials, silver fluoride is preferred for use in the iluoberyllate electrolyte resulted in undetectable polarizaother two.

TABLE IIL-BOLUBILIIIES OF CATHODE MATERIALS Solution I Solution II Solution III Temp. Solubility Temp. Solubility Temp. Solubility Material C.) (W/ (W/ i NiFg "L (P- 933- Q 9 133 AgF g 2 8 0.0004 375 0. 0022 3,5 0. 00st :ion at 200 mar/cm The electroformed anode has been The behavior of a Be anode and NiF cathode in Solu- ;ycled for ver two weeks ithout immigrating Ope -ation II at 400 C. was similar 0 that in Solution I. Pasted ion f this elgctmlyte at 370 has been quite Satis t0 electrodes discharged somewhat more completely, in that factor), However overnight Standing at temperature on a discharge at 1.9 v. and ma./cm. remained nearly flat for 30 minutes, and at 1.5 v. and 300 ma./cm. the cell ran with moderate polarization for 5 minutes. These 2R. E. Thoma, OItNL-2548, pp. 42-43 (November 1950). a R E Thoma mm D 46 (November 1959 discharge [d1scharged,] curves are shown 111 FIG. 9. CuF

R, E, Thoma mm pp 36 37 (Novembr 1 5 showed a two-step reduction at 2.4 v. and 1.8 v., very similar to its behavior in Solution I. Silver fluoride, however, gave a single step reduction at 2.6 v. compared to the twostep reduction at 2.75 v. and 1.8 v. observed in Solution I. Dischargeld] at 100 ma./cm. was accompanied by a polarization of 170 rnv. (almost all of which was IR in the solution) as shown in FIG. 10. A sheet of silver discharged at 90 ma./cm. immediately after charging ran for 2.5 minutes at 2.75 v. The stabilization of the higher voltage discharge of silver flouride will be discussed fur ther below.

Since solubilities were undoubtedly too high in Solution III for materials other than silver fluoride; only this compound was used as cathode. One considerable advantage of Solution III is that its phase diagram permits the widest variation in compositions at 40 C. while still remaining liquid. BeF for instance, may be varied from 52 to 78 mol. percent at this temperature. A single step discharge at 2.5 v. and 25 ma./cm. 2.4 v. and 50 ma./cm. 2.2 v. and 100 ma./cm. and 2.0 v. at 200 ma./cm. was obtained with a Be anode and silver fluoride cathode. Since the current density-voltage relationship was essentially linear from -200 ma./cm. the polarization is apparently entirely IR, either in the electrolyte or in the electrode and should be improved by better design and construction.

EXAMPLE 2 By procedures analogous to those described in Example 1, a fluozirconate cell was tested. The anode was zirconium sheet, the cathode was pasted nickel fluoride, and the electrolyte comprised 10 mole percent sodium fluoride, 48 mole percent potassium fluoride, and 42 mole percent zirconium fluoride. The open circuit voltage of this cell was 1.53 volts at 420 C., and it charged and discharged at an anode current density of 20 ma./cm. at 1.45 volts. At 535 C., it charged and discharged at 200 ma./cm. at 1.00 volt. The cell was operated at temperatures of from 400 to 800 C.

What is claimed is:

1. A rechargeable thermal electrochemical cell having an anode, a cathode, and an electrolyte in contact with said anode and cathode, wherein, in the charged condition:

(a) said electrolyte comprises alkali metal fluometalate, wherein said fluometalate [metalate] is fluoberyllate [beryllate] or fluozirconate [zirconate];

(b) said anode consists essentially of a metal equal in activity to or less active in the electrornotive series than the metal of said fluomeralare [metalate]; and

(c) said cathode comprises nickel fluoride, silver fluoride, or copper fluoride.

2. The cell of claim 1 wherein said electrolyte is an alkali metal fluoberyllate [beryllate].

3. The cell of claim 2 wherein the anode is beryllium.

4. The cell of claim 3 wherein the electrolyte has a melting point between about 350 C. and about 600 C., and wherein the solubility of the cathode in said electrolyte is from about 10- to about l0- weight percent, based on weight of electrolyte.

5. The cell of claim 4 wherein the electrolyte has a melting point below about 400 C.

6. The cell of claim 4 wherein the electrolyte contains lithium fluoride, potassium fluoride, sodium fluoride, or mixtures thereof.

7. The cell of claim 5 wherein the electrolyte consists essentially of beryllium fluoride, lithium fluoride, and sodium fluoride.

8. The cell of claim 7 wherein the proportions of the components in the electrolyte are approximately 37.5 mole percent beryllium fluoride, 31.5 mole percent lithium fluoride, and 31 mole percent sodium fluoride.

9. The cell of claim 7 wherein the cathode is nickel fluoride.

10. The cell of claim 5 wherein the electrolyte consists essentially of beryllium fluoride, sodium fluoride, and potassium fluoride and wherein the cathode is silver fluoride.

11. The cell of claim 10 wherein the proportions of the components of the electrolyte are approximately 47.5 mole percent beryllium fluoride, 26.3 mole percent sodium fluoride, and 26.3 mole percent potassium fluoride.

12. The cell of claim 5 wherein the electrolyte consists essentially of beryllium fluoride and potassium fluoride, and wherein the cathode is silver fluoride.

13. The cell of claim 12 wherein the proportions of the components in the electrolyte are approximately 58 mole percent beryllium fluoride and 42 mole percent potassium fluoride.

14. The cell of claim 1 wherein the electrolyte is an alkali metal fluozirconate.

15. The cell of claim 14 wherein the anode is zirconium.

16. The cell of claim 15 wherein the electrolyte has a melting point between about 350 C. and about 600 C., and wherein the solubility of the cathode in the electrolyte is from about 10* to about 10* weight percent, based on weight of electrolyte.

17. The cell of claim 16 wherein the electrolyte has a melting point between about 400 C.

18. The cell of claim 17 wherein the electrolyte consists essentially of sodium fluoride, potassium fluoride, and zirconium fluoride.

19. The cell of claim 18 wherein the proportions of the components of the electrolyte are approximately 10 mole percent sodium fluoride, 48 mole percent potassium fluoride, and 42 mole percent zirconium fluoride.

20. The cell of claim 18 wherein the cathode is nickel fluoride.

21. The cell of claim 19 wherein the cathode is nickel fluoride.

References Cited The following references, cited by the Examiner, are of record in the patented file of this patent or the original patent.

UNITED STATES PATENTS 3,591,418 7/1971 Sutula l3683 T 3,350,230 10/1967 Pannenberger et a1. l3686 F 3,488,221 l/l970 Shimatake et al. 136-6 LF FOREIGN PATENTS 1,070,937 6/1967 Great Britain 136l53 ALLEN B. CURTIS, Primary Examiner C. F. LEFEVOUR, Assistant Examiner US. Cl. X.R. 136153, 

1. A RECHARGEABLE THERMAL ELECTROCHEMICAL CELL HAVING AN ANODE, A CATHODE, AND AN ELECTROLYTE IN CONTACT WITH SAID ANODE AND CATHODE, WHEREIN, IN THE CHARGED CONDITION: (A) SAID ELECTROLYTE COMPRISES ALKALI METAL FLUOMETALATE, WHEREIN SAID FLUOMETALATE (METALATE) IS FLUOBERYLLATE (BERYLLATE) OR FLUOZIRONATE (zIRCONATE); (B) SAID ANODE CONSISTS ESSENTIALLY OF A METAL EQUAL IN ACTIVITY TO OR LESS ACTIVE IN THE ELECTROMATIVE SERIES THAN THE METAL OR SAID FLUOMETALATE (METALATE); AND (C) SAID CATHODE COMPRISES NICKEL FLUORIDE, SILVER FLUORIDE, OR COPPER FLUORIDE. 